The thyristor is a high power semiconductor switch that permits large currents to be switched at high voltages. The thyristor has four semiconducting layers. Typically, the two outer layers are heavily doped layers, while the inner two are lightly doped. Adjacent layers are oppositely doped from their neighbors, forming a number of semiconductor junctions therebetween.
The thyristor is turned on when carriers enter one of the inner layers. Typically, this is performed by injecting a small gate current pulse into one of the inner layers. Where the gate current is injected over only a portion of the inner layer, the current through the thyristor does not reach a maximum value until the entire layer is conducting. The time taken for the current to spread laterally to fill the layer is limited by the local carrier gradient. The device only reaches full current capacity after the current has spread sufficiently to uniformly saturate the device. One approach to reducing the turn on time is to inject the gate current over a large area of the thyristor, which necessitates a high degree of interdigitization between the gate electrode and the cathode electrode. An increased gate electrode area results in a reduced active area on the wafer for carrying the high current.
Another method of turning on a thyristor is to rapidly change the anode voltage with time. When the anode voltage is rising rapidly a current flows through the device because of its capacitance. This current can be sufficient to turn the device on. This triggering method, however, is less controllable than gate current injection because the device can turn on in a small area, due to inhomogeneities in the semiconductor. The entire anode current flowing through the small conducting region produces excess power dissipation leading to device failure.
A third method for turning on a thyristor is to create charge carriers in the center junction through the absorption of light. Past attempts at light activation of a thyristor have included illuminating a portion of the thyristor with light transmitted from the end of a fiber. This also results in the turn-on time of the thyristor being limited by the lateral spreading of the on-region.
The illumination of a thyristor through the side of a fiber results in the illumination of a greater volume of the device, resulting in the generation of charge carriers throughout a larger fraction of the device, and so the device can reach its maximum current carrying capability more quickly.